Move module prototypes higher in the Metal output.
Move module prototypes higher in the Metal output.

This fixes a code ordering issue when an SkSL module function uses an
out parameter.

Change-Id: Ieaf641301c2a4abd42239f35cf71188ef076d210
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/578636
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>
Auto-Submit: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia